1. Field of the Invention
The present invention relates to well logging techniques, wherein a sonde is lowered in a well or borehole to perform measurements from which are derived information about the composition and/or the physical structure of the earth formation surrounding the borehole, or the borehole fluid, or the annulus including casing and cement located between the borehole wall and the formation.
2. The Related Art
A logging sonde usually comprises a hollow cylindrical body or housing made out of metal, and in which are disposed the sensitive devices designed to perform the measurements and transmit the data via the cable to the surface. The housing is designed to contain and to protect such sensitive devices from the hostile environment of the borehole, including high temperature, high pressure and chemical/mechanical aggressions.
In order to counter balance the extremes forces which apply against the external surface of the housing, or at least to reduce the differential pressure between inside and outside of the housing, it is commonplace to fill the housing with a fluid, usually oil. Such oil is usually referred to as "hydraulic oil". Another purpose of hydraulic oil is, in electrical type sondes, to electrically insulate the electrical sensitive devices, such as coils, with respect to the housing wall and/or other conductive elements or devices disposed in the housing and which might interfere electrically with the coils. In sonic type sondes, a further purpose of hydraulic oil is to enhance propagation of acoustic waves. A still further purpose of hydraulic oil is to actuate hydraulic equipment such as pumps or pistons in logging tools designed to extract a core from the formation, as depicted in U.S. Pat. Nos. 4,714,119 and 3,596,511.
Furthermore, a sonde usually includes at each of its ends a pressure bulkhead including an electrical connector designed to electrically link the sensitive devices located inside of the housing to the electronic cartridge disposed at the top of the sonde and which is linked to the cable (at the upper end) or to another sonde (at the lower end).
These connectors must fulfill opposite requirements, since they must be fluidproof, electrically and mechanically reliable as well as easy to manipulate. These requirements are difficult to conciliate considering that the working conditions encountered in a borehole are tough.
Nevertheless, sondes often face electrical problems at the level of the electrical connector for the following reason. In spite of all precautions taken avoid leaks in the housing, some borehole fluid penetration may occur. These unwanted leaks mainly result from the repeated use of the tool in a high pressure environment which imposes high flexing stresses on seals. Practically, the hydraulic oil cannot be maintained 100% pure, meaning there might be some water and/or borehole fluid mixed in the oil. This is critical since hydraulic oil is partly miscible with water and borehole fluid is most times, if not always, conductive, and may also include some tiny metal fragments or debris resulting e.g. from wear. Water, borehole fluid and debris entering through unwanted leaks in the housing and contaminating the hydraulic oil, are heavier than the hydraulic oil. They sooner or later find their way by gravity to the bottom of the sonde, where the electrical connectors are located, and thus affect the electrical insulation at the level of the connector. This may put in jeopardy the operation of the sonde.
Accordingly, there is a need in the logging industry for improving reliability of sondes and avoid the above referred problems.